Stress embrittlement testing



Sept 23, 1969 H. E. BUSH STRESS EMBRITTLEMENT TESTING Filed March 8, 1967 nited States Patent 3,468,160 STRESS EMBRITTLEMENT TESTING Harley Eugene Bush, Houston, Tex., assignor to National Lead Company, New York, N.Y., a corporation of New Jersey Filed Mar. 8, 1967, Ser. No. 621,686 Int. Cl. G01n 17/00 U.S. CI. 73-86 Claims ABSTRACT OF THE DISCLOSURE This invention provides a novel steel test coupon for rapid and highly reproducible testing of fluids for the determination of the liability to stress corrosion cracking and embrittlement for high-strength steels in the test fluid; and also provides a method for producing the test coupon in quantity, whereby slight variations in the stock used for the purpose of the test coupon are largely equalized out. The coupons are prepared by mutual compression at right angles of cylindrical bearing rollers, so as to produce highstressed, generally circular depressions on the cylinder sides thereof.

This invention relates to a novel test coupon for testing fluids in which high-strength steels may be subject to stress corrosion cracking, and to a process for making the said coupon.

With the increasing use in industry of high-strength steels in environments where pipes, tubing, rods, structural members, and the like made from such steels are in contact with aqueous fluids of various kinds, a rather special type of failure of the steel under certain conditions has come to light, and only relatively recently has found its explanation in the light of present-day metallurgical and electrochemical knowledge. The phenomenon is generally known as stress corrosion cracking, and as stress embrittlement, and is to be expected when steel is in an environment wherein hydrogen ions are reduced to atomic hydrogen at the surface of the steel, and when at the same time, the steel is under stress at the point concerned, and further, when the steel is of a high-strength alloy. A further probable contributing factor is the presence of sulfide ion, or alternatively, arsenic and possibly other less common ions. It will be recognized that such conditions are commonly present in oil field practice, where the various steel members concerned with drilling, producing, and workover of oil and gas wells are in a subsurface, aqueous environment. The phenomenon which is observed is that under these conditions, the steel may crack, split and fail often with astonishing rapidity. The explanation appears to be that the hydrogen atoms, as produced in a nascent state by the electrochemical reduction of hydrogen ions in the aqueous contact liquid, instead of immediately combining to form diatomic hydrogen molecules, are instead inhibited from so doing by the presence of the sulfide ion which acts as a negative catalyst and migrate in the monatomic state to the interior of the steel; this migration probably being facilitated by the state of stress of the steel. Eventually the hydrogen atoms, now in the interior of the steel piece, combine to form ordinary diatomic hydrogen gas, and the pressure built up eventually becomes so great that the steel is cracked, the stressed state again probably facilitating localization of disruptive stresses in the submicroscopic cracks, dislocations, discontinuities, and the like in the structure of the steel.

It may be observed parenthetically that alloys of other metals such as aluminum, magnesium, and the like are also subject be embrittlement. The present invention, however, is not addressed to embrittlement in nonferrous metals.

It is become of increasing importants to be able to make bench tests on fluids obtained from the field environment of steel susceptible to such failure, in order that remedial measures may be taken promptly so as to preserve the steel structures intact. The general procedure is to prepare a test specimen (coupon) of high-strength steel having such a configuration that it can be immersed in the test liquid in a stressed state. If the various parameters present combine to favor stress corrosion cracking, then in the general case, the test specimen will show signs of failure in a reasonably short time, which may be of the order of magnitude of an hour or two, or of a fraction of an hour. A variety of test specimen configurations have been proposed and used. Some are described in the book, Stress Corrosion Cracking and Embrittlement, W. D. Robertson, editor, John Wiley, New York, 1956. One type consists of a section of rod, generally turned down somewhat in a middle portion, which is maintained under externally applied tension during the test period. Another type consists of a section of a piece of steel bent in a generally C shape with a bending stress maintained by a clamp or nutand-bolt arrangement, or the like. The known configurations of test specimens are normally cumbersome and do not readily adapt themselves to the carrying out of a large number of tests in a short time with the simplest of laboratory equipment and at a low cost.

An object of the present invention is to provide a novel, reproducible, fast-acting embrittlement test coupon, together with a method of producing such a coupon in large numbers at low cost with simple equipment and in a reproduction fashion.

Other objects of the invention will appear as the description thereof proceeds.

In the drawings, FIGURE 1 is a perspective view of a stock piece prior to its formation into the inventive embrittlement test coupon.

FIGURE 2 is a perspective view showing my embrittlement test coupon, while FIGURE 2a is a similar view of such a coupon after having been used in testing in embrittlement solution.

FIGURE 3 is a side view partially in section, and partially fragmentary, showing the method of forming the coupon in accordance with the invention.

FIGURE 4 is a fragmentary sectional view taken as shown by the arrows in FIGURE 3.

FIGURE 5 is a view taken similarly to FIGURE 4, but showing the configuration after forming a pair of test coupons.

FIGURE 6 is a view, partially in section, showing one of my inventive test coupons during an embrittlement test.

Generally speaking, and in accordance with an illustrative embodiment of my invention, I provide two sections of cylinders having chamfered ends, and preferably of high-strength steel corresponding to American specification A.I.S.I. S5, having a Rockwell C hardness of 55 to 57, and preferably from about to /2 inch in diameter, and I then place these round steel bars in contact so that the axes of the cylinders are at right angles, the cylindrical surfaces being in contact at one point. I then force the two cylinders against each other at their point of contact, using a total force of from 30,000 to 50,000 pounds, although I prefer a total force of about 40,000 pounds. The result of mutually pressing the two cylinder pieces together is that a flattish area of generally circular periphery and of diameter about .50 to 0.70 of the cylinder diam eter is formed in the center of each piece. The test piece or coupon is then ready for use, or it may be placed in a suitable noncorrosive environment and stored for later use at any desired time.

The sections of cylinders may have any desired length, although the length should be at least one-and-one-half times the diameter to prevent undesired deformation during compression. Long sections, e.g., three or four or more times the diameter, may be used, but are wasteful of material and also tend to mask the failure manifestations. I prefer a cylinder length of from about /1 to one inch in length (I find best a diameter of inch and a length of inch). The cylindrical sections preferably have'chamfered ends, as shown in the drawings.

The flattened area should be within the diametrica limits already given. Less than 0.50 corresponds to so little stress-deformation that embrittlement testing times are prolonged and false negatives may occur. More than 0.70 corresponds to so much stress deformation that spontaneous cracking may occur, leading to false positives.

Turning now to the drawings, FIGURE 1 shows one cylindrical piece as described above, and FIGURE 3 shows two such pieces, 10 and 11, juxtaposed in the manner already described. These are squeezed together by the action of the anvils 12 and 13 of a hydraulic press 14. In order to maintain the cylinder pieces 10 and 11 in proper relationship during the compression, I prefer to make use of jigs 16 and 17 which are placed on the two anvils of the press. Each jig has a hemicylindrical channel 18 and 19, and locating pins 20, 21 and 22 to prevent displacement of the cylinder pieces. The jigs 16 and 17 should be made out of steel having suflicient hardness and tensile strength so that the jigs will not be themselves deformed during compression.

I then operate the hydraulic press 14 so as to compress the two jigs 16 and 17 together at a total force of, for example, 40,000 pounds. FIGURE 4 shows the point of contact between two cylinder pieces 10 and 11 prior to compression, while FIGURE 5 shows in cross-section a line of contact between the two pieces after compression. The deformation takes place substantially instantly, and much more quickly than the operating time of ordinary hydraulic presses.

The mutual interaction of the two test coupons 10 and 11 is equal and in a sense symmetrical so that each test piece is deformed in substantially the same fashion as it is made, and after removal from the press, the top piece is indistinguishable from the bottom. There are, of course, inevitable slight variations between any two individual pieces, no matter how carefully they have been fabricated and heat-treated, and one advantage of my inventive procedure is that for any given pair of test pieces, the weaker one will necessarily be deformed somewhat more than its partner so that the test coupons themselves are more uniform than the starting pieces before compression. This leads to remarkable reproducibility in embrittlement tests employing my inventive coupons. Variations occur even with high-grade roller-bearing rollers, which are a convenient source of the cylinders.

The configuration of the stressed area on an inventive coupon, as for example the coupon 25 shown in FIGURE 2, is that of an approximately flattened circle 26, although it is indented more than a fiat, in the strict sense, would be. The exact shape may be perceived from the side view appearing in FIGURE 6 and the sectional view appearing in FIGURE 5.

In order to carry out a test with an inventive coupon, it is merely necessary to immerse the coupon in a sample of the liquid to be tested. A typical test is shown in partial cross-section in FIGURE 6 wherein is a beaker, 31 is the fluid to be tested, which, for example, may be a subsurface liquid from an oil well drilling operation, soil water adjacent to pipe line, ocean water adjacent a marine installation, or the like; and, 25 is one of my inventive test coupons. I find it convenient to keep the coupon from contacting the sides and bottom of the vessel, for which I employ a pair of O-rings 32 and 33. These also induce concentration-cell effects, further simulating the field environment. These are formed of a non-conductive elastomcric substance, such as natural or synthetic rubber, and are slipped over the-cylindrical test coupon, but preferably not contacting the flattened area. If the fluid has embrittling characteristics, then the test coupon 25 will fail by cracking and even splitting. A typical failure is shown in FIGURE 2a, where the test coupon has been split nearly in two by the embrittlement phenomenon. The time for failure to occur is extremely rapid, especially when compared to many prior art methods of testing. For example, a test coupon prepared as has been described will fail in approximately 60 seconds in 5 percent aqueous hydrochloric acid solution, and approximately 10 seconds in 37.6 percent hydrochloric acid solution, all at room temperature of F. Again, in the relatively mild environment of distilled water with 50 parts per million sulfide ion added, my inventive test coupons will fail in from A or /2 hour. The advantages of such a rapid-acting test coupon for carrying out routine, field, and screening tests are obvious.

It will be clear that while I have described my inventive test coupon and its method of manufacture with the aid of specific examples, nevertheless considerable variation is possible without deviating from the broad scope of my invention, as defined by the claims which follow.

Having described my invention, I claim:

1. The process of producing a pair of steel embrittlement test coupons which comprises the steps of juxtaposing two cylindrical sections of round steel bar in pointto-point contact with the axes of said cylinders at right angles, and thereafter compressing said cylinders together with a force sufiicient to produce flattened areas having diameters of from about 0.50 to about 0.70 times the diameter of said cylinders about the point of contact, and thereafter separating such cylinders so as to provide separate test coupons for use.

2. The process in accordance with claim 1 wherein said cylinder sections are constituted of steel of Rockwell hardness approximately 55 to 57.

3. The process in accordance with claim 2 wherein said cylinders are from about to /2 inch in diameter.

4. The process in accordance with claim 3 in which said cylinders have lengths of from about inch to about one inch.

5. The process in accordance with claim 4 wherein said force is from about 30,000 pounds to about 50,000 pounds.

6. The process in accordance with claim 5 wherein said steel corresponds to American specification A.I.S.I. S5.

7. The process of determining the embrittlement characteristics of a test fluid against steel which comprises the steps of immersing in a sample of said test fluid in a test coupon manufactured in accordance with claim 1, permitting it to stand for a predetermined length of time, removing said test coupon, and observing said test coupon for evidence of failure.

8. The process of determining the embrittlement characteristics of a test fluid against steel which comprises the steps of immersing in a sample of said test fluid a test coupon manufactured in accordance with claim 6, permitting it to stand for a predetermined length of time, removing said test coupon, and observing said test coupon for evidence of failure.

9. The process in accordance with claim 7 wherein a pair of non-conductive, elastomeric O-rings are emplaced on said coupon during said immersion.

10. The process in accordance with claim 8 wherein a pair of non-conductive, elastomeric O-rings are emplaced on said coupon during said immersion.

References Cited UNITED STATES PATENTS 2,348,782 5/1944 Bollee et al 73-81 2,484,279 10/1949 Folz 7386 RICHARD C. QUEISSER, Primary Examiner C. E. SNEE III, Assistant Examiner US. Cl. X.R. 72377 

